Filtering screen

ABSTRACT

A screen is described for use in a vibrating machine for separating solids from liquid material, comprising woven wire cloth of orthogonal warp and weft wires, tensioned and bonded to a support structure defining at least one rectangular opening across which the cloth extends. The orientation of the cloth is chosen so that the warp wires extend across the width (i.e. shorter dimension) of the opening(s). If the cloth has a square mesh and the warp wires are of greater cross section than the weft wires, the warp wires will extend across the width of the frame, and if the cloth has a rectangular mesh, the greater number of warp wires per unit length will also extend across the width of the frame, so that in each case warp wires will resist in use the stresses across the width of the central region of the or each opening.

FIELD OF THE INVENTION

This invention concerns screens such as are fitted to vibratingscreening machines, sometimes used as shale shakers to separate solidsfrom fluids. Such machines are of particular application in the oil welldrilling industry to separate drilling mud from base fluid afterrecovery from down-hole during drilling.

BACKGROUND

Filter screens for use in such machines are typically constructed fromwoven wire cloth. When weaving wire cloth the warp wires are those thatrun along the length of the roll of wire-cloth as it is woven, and woundaround the take-up drum, while the weft wires are those which run acrossthe width of the wire-cloth.

Square mesh wire cloth is comprised of nominally identical numbers ofwarp and weft wires per unit area, and a common wire diameter. Forexample, a 200# market grade cloth has 200 warp wires per inch and 200weft wires per inch. Both warp and weft wires are 0.050 mm in diameter.

Although there is still a desire to use wire cloth with generally squareopenings, and the use of generally square mesh as the filter media foroilfield screens is still widespread, rectangular meshes have proved tobe successful as a robust, high capacity alternative to square mesh.

Robust filter media incorporating rectangular mesh are disclosed in U.S.Pat. No. 5,944,197 and PCT Application PCT/GB2002/005018.

A rectangular mesh is normally woven with more warp wires per unitlength than weft wires per unit length, since the time taken to weave agiven length of wirecloth is dependent on the number of weft wires.

One common type of screen comprises layers of mesh bonded to a supportstructure (normally referred to as a frame) which is usually generallyflat and rectangular in shape, and which contains a number of similarlysized (normally rectangular) openings across which the screen mesh istensioned. The mesh is supported by the frame and the openings in theframe define a corresponding number of mesh covered windows forfiltering the fluid materials. The frame may be of metal but morepreferably is of a plastics material particularly GRP and preferably isreinforced internally by a wire or rod framework. Such screens will bereferred to as integral screens, that is the mesh and frame areintegrated by the bonding of the mesh to the support frame. A jig formaking integral screens in which two screens are made at the same time,is described in GB Patent Specification 2,382,037. Such a jig will bereferred to as a jig of the type described.

In operation the maximum stress on the wire cloth in such a screen isfound to occur at the middle of the longer dimension of the frame. Thissuggests that the wires running parallel to the shorter sides aresubject to greater stress than those running parallel to the longersides of the screen. Areas of maximum stress are indicated in FIG. 1,which is described more fully later.

It has also been observed in practice that the wires running parallel tothe shorter span of the mesh in such a frame often tend to fail first,which also supports the theory that these wires are subject to greaterstress.

Another common type of screen is a so-called hook-strip screen. Such ascreen consists of generally rectangular sheets of wire cloth (mesh)with hooks along two parallel sides. The sheets are attached by thehooks to a stretching mechanism in the shaker. This stretches the meshto tension the wire cloth. This is necessary to encourage good solidsconveyance across the stretched mesh in use.

In practice hook-strip screens are usually stretched over a supportwhich presents a convex upper surface to the mesh so that the mesh intension becomes convexly curved as shown in FIG. 2. In general only twoedges of the mesh include hooks, and the other two edges are not securedto the shaker. Therefore the tensioning load is applied in one directiononly. This means that if the screen is over-tensioned the wires parallelto the tensioning direction will tend to fail before the wires extendingin the perpendicular sense. In use, over-tensioning can occur due toexcessive solids build-up or any general overloading of the screen, aswell as due to any inappropriate tensioning of the mesh during set-up.

SUMMARY OF THE INVENTION

According to one aspect of the present invention an integral screen foruse in a vibrating machine for separating solids from liquid material(especially solids from drilling mud recovered during oil welldrilling), comprises woven wire cloth of orthogonal warp and weft wires,tensioned and bonded to a support structure defining a rectangularopening across which the cloth extends, wherein the orientation of thecloth is chosen so that the warp wires extend across the width (i.e.shorter dimension) of the rectangular opening and the weft wires extendacross the length (i.e. longer dimension) of the rectangular opening.

Where the rectangular opening in the support structure includes aplurality of similarly dimensioned and orientated and regularly arrangedsmaller rectangular openings, formed by a lattice of strutscriss-crossing the larger opening, the cloth is bonded to the latticestruts as well as the boundary of the larger opening, so that the warpwires are also parallel to the width (i.e. the shorter dimension) of thesmaller rectangular openings.

In the case of a rectangular mesh cloth there will be more wires perunit length across the width of the rectangular opening than there areweft wires, to resist the greater stress found to occur across the widthof the central region of the or each opening.

A cloth is thought of as having a rectangular mesh if the aspect ratioof the openings in the weave is at least 0.8:1.

In the case of a generally square mesh cloth the warp wires arepreferably selected to have a greater cross sectional size than the weftwires, which if they extend perpendicularly relative to the lengthdimension of the or each opening are able to resist the greater stressacross the width of the central region of the or each opening.

A cloth is said to have a square mesh if the openings in the weave havean aspect ratio of between 0.9:1 and 1:1.1.

Where the mesh is generally square, the larger warp wires preferablyhave a cross-sectional area of between 10% and 30% greater than thesmaller weft wires.

More preferably the larger warp wires have a cross-sectional area in therange 20% to 25 % greater than the smaller weft wires.

Typically, the larger warp wires have a cross-sectional area 22% greaterthan that of the smaller weft wires.

The wires are typically of circular cross-section.

In one generally square mesh example, the diameter of the larger warpwires is 0.046 mm, and the diameter of the weft wires is 0.036 mm, andthere may be 200 warp wires per inch and 230 smaller weft wires perinch.

In another generally square mesh example, the diameter of the wires isas before, but there are 212 larger warp wires per inch and 230 smallerweft wires per inch.

If rectangular mesh cloth is employed in the manufacture of arectangular integrated screen, it is generally believed that in order toachieve optimum solids conveyance and de-blinding the rectangularopenings in the weave should be aligned with their longer dimensionparallel to the direction of solids flow over the screen. Where thesupport frame has a plurality of regularly arranged similarly orientatedand similarly sized smaller rectangular openings, the longer dimensionsof which are parallel to the length dimension of the frame, the solidsflow will normally be parallel to the length dimension of the frame, andaccordingly it has been considered necessary for the warp wires in therectangular mesh cloth stretched over the rectangular openings, to runthe length of the rectangular openings, to achieve optimum solidsconveyance and deblinding.

A common overall frame size is of the order of 42″×30″ and if 48″ widecloth is to be used so that the weft wires extend across the 30″dimension, the cloth has to be orientated relative to the frame so thatthe 48″ width extends across the 30″ width of the frame. Accordinglythere will be a lot of waste cloth to be cut away from the two longeredges of the frame after bonding.

Trials have been undertaken to determine if this orientation ofrectangular mesh cloth does in fact make a noticeable difference to theperformance of the screen, as compared to the orthogonal orientation, inwhich the longer dimension of the rectangular openings in the weave isperpendicular to the direction of flow. In this orthogonal orientationthe longer dimensions of the openings in the weave will be perpendicularto the length dimension of the rectangular frame and that of each of therectangular windows in the frame, and the warp wires will now extendacross the width of the frame and across the width of each of thesmaller windows. No noticeable difference was found.

According therefore to another aspect of the invention in a method ofmanufacturing two integral screens side by side in a jig of the typedescribed wherein a length of woven wire cloth is laid across tworectangular frames laid side by side in the jig with longer edgesthereof abutting, the cloth is orientated so that the warp wires extendcontinuously across the two side by side screens and the weft wiresextend parallel to the longer edges of the frames, and the cloth isbonded to the frames before being severed along the join and surpluswire cloth is trimmed away from around the edges of the frame.

Thus in accordance with the method proposed by the invention in whichstandard 48″ wide woven wire cloth is to be employed, the 48″ wide clothis cut to 66″ length, and laid over the two side by side frames in thejig, with the warp wires perpendicular to the length dimension of theframes and where the frames include smaller rectangular openings, thewarp wires are therefore perpendicular to the longer dimension of eachof the smaller openings in the frames.

This not only allows less costly cloth to be used but also improves thescreen life since if the cloth has a square mesh and the warp wires havea greater cross section size than the weft wires, the stronger warpwires will extend across the width of the support frame (and the widthof each window in the frame), and if the cloth has a rectangular mesh,the greater number of warp wires per unit length will also extend acrossthe width of each support frame (and the width of each window in theframe), and therefore in each case the greater wire cross section orgreater number of wires per unit length, will resist the stresses foundto occur across the width of the central region of the or each opening.

As a further advantage, by using 48″ wide cloth, cut to 66″ lengths fromthe 48″ roll, there is much less waste cloth to be trimmed off the edgesof the frames after the cloth has been bonded to the frames, as comparedwith the use of 48″ wide cloth applied to one frame at a time, so as toproduce the mesh orientation previously believed to be desirable forrectangular mesh cloth.

Making screens in accordance with the invention contrary to thepreviously long held belief that the warp wires of rectangular meshcloth should run parallel to the direction of solids flow over thescreen, allows screens to be made with no apparent loss of performance,using low cost standard 48″ width cloth and with minimal waste cloth.

The invention thus has the advantage of enabling fully functional andlong-life screens to be made using standard 48″ wide woven wire cloth.

According to another aspect of the present invention a hook-strip screenfor use in a vibrating machine for separating solids from liquidmaterial (especially solids from drilling mud recovered during oil welldrilling) comprises a sheet of woven wire cloth having a plurality ofhooks along two opposite parallel edges of the sheet of wire cloth forattaching the two ends of the sheet to the machine, which edges areparallel to the weft wires of the weave, so that the warp wires extendbetween the edges containing the rows of hooks.

In the case of a rectangular mesh cloth in which there are more warpwires than weft wires per unit length, the greater number of warp wireswill be available to resist any over-tensioning.

Likewise, if the cloth has a square mesh, the hooks will be positionedalong the two parallel edges of the cloth between which the greatercross-section warp wires extend, which again are available to resistover-tensioning.

In each case the hooks of the hook-strip screen are used to retain thesheet of wire cloth in a shaker machine in manner know per se.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plan view of a wire mesh filter screen;

FIG. 2 is a plan view of a hook-strip screen, stretched over a radius;

FIG. 3 is a perspective view of a wire cloth roll, partly unrolled;

FIG. 4 is a plan view of a rectangular panel cut from the roll with thewarp and weft wires shown to an enlarged scale in a scrap view;

FIG. 5 illustrates the orientation of rectangular mesh openings relativeto the direction of solids flow over the screen, which hitherto has beenheld to be the preferred orientation for the openings in the weave ofsuch cloth;

FIG. 6 is a plan view of an integral rectangular screen showing howrectangular mesh cloth has (historically) been orientated relative tothe support frame;

FIG. 7 shows how two frames such as shown in FIG. 6 can be covered usinga single 66″ wide cloth using a preferred method of manufacture; and

FIG. 8 shows how a length of 48″ wide rectangular mesh wire cloth can beused in the preferred manufacturing process in which two screens aremade at the same time in a single jig.

Some examples of woven wire cloth are given below.

A standard 230 mesh screen cloth has the following features:

Standard 230#

warp count 230 per inch warp diameter 0.036 mm weft count 230 per inchweft diameter 0.036 mm

Nominal Aperture size—0.074×0.074 mm

A screen has been manufactured in accordance with the invention, whichhas larger warp wires than weft wires.

The modified cloth has the following features:

Modified 230#

warp count 200 per inch warp diameter 0.046 mm weft count 230 per inchweft diameter 0.036 mm

Nominal Aperture size—0.081×0.074 mm

The wires of the modified 230# mesh cloths provide a slightly elongatedwire aperture (having a 1:1.1 aspect ratio). This does not compromisethe cut point significantly. The overall nominal cut point would be 76.3rather than 74 (by the equivalent spheres method).

The conductance of the modified mesh is probably decreased from 1.17kD/mm to 1.07 kD/mm. However this is offset by the fact that the warpwires have 22% greater cross-sectional area, which significantlyprolongs the life of the screen.

An alternative modified 230# mesh cloth has the following attributes:

warp count 212 per inch warp diameter 0.046 mm weft count 230 per inchweft diameter 0.036 mm

Nominal Aperture size—0.074×0.074 mm

Thus the wire apertures are square.

Up to 85% of looms that are used for weaving wire-cloth in the worldtoday weave cloth which is up to a maximum of 48 inches in width (i.e.in the weft direction). This is illustrated in FIG. 3. Therefore 48 inchwide wire-cloth is widely available and is relatively cheap because ofthis. The other 15% of looms are designed to weave a variety of greaterwidths, such as up to 60 inches, 66 inches and 72 inches. These greaterwidths however are as a consequence much scarcer than standard 48 inchwide cloth, and can be up to four times as expensive per square metre.

As described above, a wire cloth having a generally square weave (ormesh), contains substantially as many weft wires per unit length asthere are warp wires per unit length. Where the cloth is to have arectangular mesh case there are usually more warp wires per unit lengththan weft per unit length, because the time to weave a roll ofwire-cloth is dependant upon the number of weft wires. A 48″ widerectangular mesh wire cloth is illustrated in FIG. 4.

If rectangular mesh cloth is employed in the manufacture of arectangular integrated screen, it is generally believed that in order toachieve optimum solids conveyance and de-blinding the rectangles in themesh should be aligned with their longer dimension parallel to thedirection of solids flow over the screen, as shown in FIG. 5. Where theframe has a plurality of regularly arranged similar sized smallerrectangular openings, the longer dimensions of which are parallel to thelength dimension of the frame, the solids flow will normally be parallelto the length dimension of the frame, and this means that for arectangular mesh cloth stretched over rectangular openings the warpwires should run the length of the rectangular openings, as shown inFIG. 6.

A common screen size is of the order of 42″×30″ and if 48″ wide cloth isto be used so that the warp wires extend across the 30″ dimension thecloth has to be orientated relative to the frame so that the 48″ widthextends across the 30″ width of the frame, and there will be a lot ofwaste cloth to be cut away from the two longer edges of the frame afterbonding.

In fact, such screens are preferably manufactured using a jig asdescribed in GB Patent 2,382,037. This jig essentially requires a singlesheet of wire cloth 66″ by 48″ to be laid over two frames, arranged sideby side in the jig as illustrated in FIG. 7.

However if warp wires are to traverse the 30″ dimension of the twoframes, 66″ wide woven wire-cloth will be required if a single sheet ofwire-cloth is to be stretched over and bonded to the two side by sideframes, a relatively small amount of surplus cloth has to be trimmedfrom the edges of the frame to complete the process, in the mannerdescribed in GB 2,382,037. However 66″ wide wire cloth is expensive.

Trials have been undertaken to determine if this orientation ofrectangular mesh cloth (see FIGS. 6 and 7) does in fact make anoticeable difference to the performance of the screen, as compared tothe orthogonal orientation, in which the longer dimension of theopenings in the rectangular mesh is perpendicular to the direction offlow and therefore perpendicular to the length dimension of arectangular frame having rectangular windows such as in FIG. 6. Nonoticeable difference was found.

FIG. 8 illustrates the method of making a screen as proposed by thepresent invention which allows 48″ wide woven wire cloth to be employed.Here the 48″ wide cloth is cut to 66″ length, and laid over the two sideby side frames in the jig, with the warp wires perpendicular to thelength dimension of the frames and therefore perpendicular to the longerdimension of each of the smaller rectangular openings in the frames.

This not only allows the less costly cloth to be used but also improvesthe screen life since if the cloth has a square mesh, and the warp wireshave a greater cross section size than the weft wires, the stronger warpwires will extend across the width of the support frame, and if thecloth has a rectangular mesh, the greater number of warp wires per unitlength will also extend across the width of the support frame andtherefore in each case the greater wire cross section or greater numberof wires per unit length, will resist the stresses found to occur acrossthe width of the screen.

As a further advantage, by using 48″ wide cloth, cut to 66″ lengths fromthe 48″ roll, there is much less waste cloth to be trimmed off the edgesof the frames after the cloth has been bonded to the frames, as comparedwith the use of 48″ wide cloth applied to one frame at a time, so as toproduce the mesh orientation previously believed to be desirable forrectangular mesh cloth.

Making screens in accordance with the invention contrary to thepreviously long held belief that the warp wires of rectangular meshcloth should run parallel to the direction of solids flow over thescreen, allows screens to be made with no apparent loss of performance,using low cost standard 48″ width cloth and with minimal waste cloth.

The invention thus has the advantage of enabling fully functional andlong-life screens to be made using standard 48″ wide woven wire cloth.

1. An integral screen for use in a vibrating machine for separatingsolids from liquid material, comprising a support structure comprising arectangular support frame defining a first rectangular opening having awidth shorter in dimension than a length, and woven wire cloth oforthogonal warp and weft wires, the first rectangular opening in thesupport structure including a plurality of similarly dimensioned,similarly orientated and regularly arranged smaller rectangular openingsor windows, formed by a lattice of struts criss-crossing the firstopening, wherein the cloth is bonded to the support structure, beingbonded to the lattice struts as well as the boundary of the firstopening, with the cloth extending in a tensioned state across the firstopening, the cloth has rectangular openings in the weave, having agreater number of warp wires per unit length than there are weft wiresper unit length, and the orientation of the cloth is such that the warpwires extend across the width (i.e. shorter dimension) of the firstrectangular opening and the weft wires extend across the length (i.e.longer dimension) of the first rectangular opening, with the warp wiresalso being parallel to the width dimension (i.e. the shorter sides) ofthe smaller rectangular openings.
 2. A screen as claimed in claim 1wherein the warp wires have a cross-sectional area of between 10% and30% greater than the weft wires.
 3. A screen as claimed in claim 2wherein the warp wires have a cross-sectional area in the range 20% to25% greater than the weft wires.
 4. A screen as claimed in claim 3wherein the warp wires have a cross-sectional area 22% greater than thatof the weft wires.
 5. A screen as claimed in claim 1 wherein the wiresare of circular cross-section.
 6. A screen as claimed in claim 5 whereinthe diameter of the larger warp wires is 0.046 mm, and the diameter ofthe weft wires is 0.036 mm.
 7. An integral screen for use in a vibratingmachine for separating solids from liquid material, comprising a supportstructure comprising a rectangular support frame defining a firstrectangular opening having a width shorter in dimension than a length,and woven wire cloth of orthogonal warp and weft wires, the firstrectangular opening in the support structure including a plurality ofsimilarly dimensioned, similarly orientated and regularly arrangedsmaller rectangular openings or windows, formed by a lattice of strutscriss-crossing the first opening, wherein the cloth is bonded to thesupport structure, being bonded to the lattice struts as well as theboundary of the first opening, with the cloth extending in a tensionedstate across the first opening, the cloth has generally square openingsin the weave, the warp wires all have a greater cross-sectional areathan all the weft wires, and the orientation of the cloth is such thatthe warp wires extend across the width (i.e. shorter dimension) of thefirst rectangular opening and the weft wires extend across the length(i.e. longer dimension) of the first rectangular opening, with the warpwires also being parallel to the width dimension (i.e. the shortersides) of the smaller rectangular openings.
 8. A screen as claimed inclaim 7 wherein the warp wires have a cross-sectional area of between10% and 30% greater than the weft wires.
 9. A screen as claimed in claim8 wherein the warp wires have a cross-sectional area in the range 20% to25% greater than the weft wires.
 10. A screen as claimed in claim 9wherein the warp wires have a cross-sectional area 22% greater than thatof the weft wires.
 11. A screen as claimed in claim 7 wherein the wiresare of circular cross-section.
 12. A screen as claimed in claim 11wherein the diameter of the larger warp wires is 0.046 mm, and thediameter of the weft wires is 0.036 mm.
 13. A vibrating machine forseparating solids from liquid material, the machine including anintegral screen over which solids flow in a specified direction, thescreen comprising a support structure comprising a rectangular supportframe defining a first rectangular opening having a width shorter indimension than a length, and woven wire cloth of orthogonal warp andweft wires, the first rectangular opening in the support structureincluding a plurality of similarly dimensioned, similarly orientated andregularly arranged smaller rectangular openings or windows, formed by alattice of struts criss-crossing the first opening, wherein the cloth isbonded to the support structure, being bonded to the lattice struts aswell as the boundary of the first opening, with the cloth extending in atensioned state across the first opening, the cloth has rectangularopenings in the weave, having a greater number of warp wires per unitlength than there are weft wires per unit length, and the orientation ofthe cloth is such that the warp wires extend across the width (i.e.shorter dimension) of the first rectangular opening and the weft wiresextend across the length (i.e. longer dimension) of the firstrectangular opening, wherein the weft wires of the cloth are alignedwith said specified direction of solids flow over the screen, with thewarp wires also being parallel to the width dimension (i.e. the shortersides) of the smaller rectangular openings.